This invention relates to the art of earth bore hole drilling, and more particularly, to a compacted material which may be added to a drilling fluid to combat or prevent lost circulation of the drilling fluid when porosity, fractures and highly permeable vugular zones in the earth strata are encountered.
In earth hole boring operations such as oil well, gas, mineral, water well drilling and the like, the drilling bit passes through various solid and fractured or vugulated porous earth strata as it descends toward the desired depth. Drilling muds are commonly employed in order to lubricate and cool the drilling bit and act as a transport medium to remove drilled earth solids from the drilling bit and bore hole to the surface. When the drilling bit passes through the porous, fractured or vugular strata such as sand, gravel, shale, limestone and the like, the hydrostatic pressure caused by the vertical column of drilling fluid exceeds the ability of the surrounding earth formation to support this pressure. As a consequence drilling fluid is lost to the formation and fails to return to the surface. This loss may be any fraction up to 100% loss of the total circulating drilling fluid volume. This condition is known in the art as lost circulation.
In order to combat or prevent lost circulation, it has been common in the past to add any number of materials to the drilling fluid which act to reduce or prevent flow of the drilling fluid outwardly in a porous stratum thereby arresting a lost circulation condition. These materials are commonly referred to as lost circulation materials. Such prior, known, lost circulation materials include ground oyster shells, wood fiber, asbestos, chicken feathers, popped popcorn, straw, chopped rope, bark chips, ground cork, ground rubber tires, shaped plastic pellets, ground paper, chopped cellophane, mica, cottonseed hulls and gelling agents such as clay. Each of these materials attacks the problem in a different way such as by physically blocking or bridging the pores as with solid materials such as cork, rubber, plastic, etc., forming a fibrous mat such as with asbestos, straw, rope, etc. and consolidating drilling fluid material within a porous structure such as by gelling or matting with paper and combinations of these actions. While these materials are often quite effective, there is considerable expense involved in transporting, manufacturing and using such materials.
One inexpensive material which has been used is paper. U.S. Pat. No. 2,064,936 discloses the use of a slurry of ground paper as a lost cirulation material. Many problems are associated with the use of paper as a lost circulation material however. Paper does not wet in easily and thus there are problems associated with dispersing paper in a drilling fluid. Also, the bulk density of paper is very low (on the order of two to three pounds per cubic foot) and thus, large amounts, i.e. many truckloads of paper, are needed in order to arrest a single lost circulation condition in a well bore. The low bulk density also creates a wellsite storage problem.